1. Field of the Invention
The present invention relates to a new process for the manufacture of a lactitol syrup.
2. Description of the Related Art
4-O-beta-D-Galactopyranosyl-D-glucitol, commonly called lactitol, is a polyol which is of great interest because is more chemically stable and lower in calories than sucrose, while advantageously possessing the property of being suitable in the diet of diabetics. Furthermore, lactitol has the characteristic feature of not being cariogenic, which paves the way, and has already done so, for it in multiple applications in industry, especially in the food and pharmaceutical industries.
Lactitol is generally marketed in the form of a crystalline powder. Its crystallography is particularly complex if reference is made to its literature, from which it is apparent that this polyol can crystallize in different monohydrate, dihydrate or even trihydrate anhydrous forms. Also, it should be noted that lactitol can crystallize in different states of hydration and in forms which are stable to a greater or lesser degree and that several crystalline forms generally exist for this polyol for the same state of hydration.
However, the number of known crystalline forms of lactitol is in no way comparable to that known for other sugars or polyols. For example, only one crystalline form is known to date for sucrose, xylitol, erythritol or maltitol and only two or three crystalline forms for, for example, dextrose, lactose, maltose and mannitol.
Although not wishing to be bound by any particular theory, this is believed to explain why, in the case of lactitol, it has always been difficult, contrary to other sugars and polyols, to obtain essentially a single crystalline form uncontaminated by other forms. To date, controlling the crystallization remains unsatisfactory as demonstrated, in particular, in the recently published documents WO-A-92/16542 and JP-A-2,255,694 or WO-A-90/06317.
In a number of cases, and regardless of the lactitol crystalline form finally obtained, the lactitol is redissolved with other polyols so as to form a lactitol syrup which is increasingly used as sweetening composition or as texturing agent in products intended to be ingested by men or animals.
"Products intended to be ingested by man or animals" which may also be referred to as "edible products" is understood to mean products intended for ingestion and for oral administration, such as various food products such as confectionery, pastries, creams, beverages, jams, sauces, ice creams or ice cream-based desserts, prepared animal fodder, as well as pharmaceutical, dietetic or health care products such as for example elixirs, cough syrups, lozenges or tablets, chewy pastes, chewing gums, pastilles, oral health solutions, dentifrices in paste, gel or liquid form or veterinary products.
In most of these applications, the lactitol is very often combined with starch hydrolysates or with glucose syrups, which are hydrogenated. In the remainder of the text of the specification, the terms glucose syrup or starch hydrolysate may be used interchangeably. Likewise, the terms inulin syrup or inulin hydrolysate may be used interchangeably. The mixture thus prepared is generally used to replace the sucrose/glucose syrup pair. Indeed, by virtue of its taste, its texture, its organoleptic, low-calorie and non-cariogenic qualities, as well as its characteristics of hardness, melting and the like, such a mixture exhibits characteristics which are advantageously comparable to those of traditional products sweetened with sucrose and glucose syrups.
To take only one recent example of the use of a lactitol/hydrogenated starch hydrolysates or hydrogenated glucose syrups mixture, there may be mentioned the document U.S. Pat. No. 5,527,554 which describes a deep-frozen dessert with a reduced calorie content comprising from 0 to 12% fat, from 5 to 15% lactitol, from 5 to 15% of a hydrogenated starch hydrolysate, from 7 to 17% dehydrated milk, from 0.01 to 0.5% of an intense sweetener, and water.
The raw material for producing the lactitol is lactose or 4-O-beta-D-galactopyranosyl-D-glucose. Lactitol is obtained industrially by hydrogenation of a solution of lactose at about 100.degree. C. over a Raney nickel catalyst and at a hydrogen pressure of about 40 bar. Because of the low solubility of lactose (compared with glucose), the hydrogenation of a lactose solution can only be achieved, under satisfactory conditions, at low concentrations of the order of 300 to 400 g/l.
Indeed, in practice, the hydrogenation of a lactose solution occurs at most at around 30% dry matter content because, above this value, problems of viscosity due to a less efficient stirring cause a drop in the hydrogenation yield.
This limit to the initial lactose concentration has in fact been demonstrated by Linko et al. in "Carbohydrate Sweeteners in Food and Nutrition (1980), Eds Koivistoineu, P. and Hyvonen, L., Academic Press, New York, 243-57". These authors determined that catalytic hydrogenation over Raney nickel of a lactose solution was optimum when a lactose solution at 30% dry matter content was used as starting material and when the procedure was carried out at 100.degree. C. at a pressure of about 90 bar.
It could be thought that this initial lactose concentration limit of about 30% dry matter content could be easily increased by carrying out the hydrogenation at higher temperatures and pressures. But these same authors, in the same document, have shown that under higher temperature and pressure conditions, the lactitol yield is much lower because the hydrogenation is accompanied by the formation of secondary products. The latter are, on the one hand, lactulose which results from the isomerization of lactose, and on the other hand, lactose and glucose which result from the hydrolysis of lactose, all these secondary products being, moreover, hydrogenated into the corresponding polyols.
From the preceding text, it can be understood that the industrial manufacture of crystallized lactitol exhibits several disadvantages, including:
that of not being very efficient because of the relatively low dry matter content of the starting product, close to 300 g/l, which is necessary to obtain the highest possible hydrogenation efficiency; PA1 that of not being very selective in the crystalline form obtained because of the unsatisfactory control of the crystallization of the lactitol. PA1 the volumes to be treated are much smaller than in prior processes, PA1 the energy required for the evaporation of the water is substantially reduced, PA1 the high osmotic pressures caused by the high concentration of the syrups used protect them from any microbial contamination, PA1 it is possible to prepare a ready-for-use lactitol syrup without the need to use an expensive step for crystallization of the lactitol which is known, in addition, to be unsatisfactorily controlled.